In the description of the background of the present invention that follows reference is made to certain structures and methods, however, such references should not necessarily be construed as an admission that these structures and methods qualify as prior art under the applicable statutory provisions. Applicants reserve the right to demonstrate that any of the referenced subject matter does not constitute prior art with regard to the present invention. Titanium-based carbonitride alloys, so called cermets, are produced by powder metallurgical methods. Compared to WC—Co based materials, cermets have excellent chemical stability when in contact with hot steel, even if the cermet is uncoated, but have substantially lower strength. This makes them most suited for finishing operations, which generally are characterized by limited mechanical loads on the cutting edge and a high surface finish requirement on the finished component. Cermets comprise carbonitride hard constituents embedded in a metallic binder phase generally of Co and Ni. The hard constituent grains generally have a complex structure with a core, most often surrounded by one or more rims having a different composition. In addition to Ti, group VIA elements, normally both Mo and W, are added to facilitate wetting between binder and hard constituents and to strengthen the binder phase by means of solution hardening. Group IVA and/or VA elements, e.g. —Zr, Hf. V, Nb, and Ta, are also added in all commercial alloys available today.
Cermets are produced using powder metallurgical methods. Powders forming binder phase and powders forming hard constituents of cermets are mixed, pressed and sintered. The carbonitride forming elements are added as simple or complex carbides, nitrides and/or carbonitrides. During sintering the hard constituents dissolve partly or completely in the liquid binder phase. Some, such as WC, dissolve easily whereas others, such as Ti(C,N), are more stable and may remain partly undissolved at the end of the sintering time. During cooling the dissolved components precipitate as a complex phase on undissolved hard phase particles or via nucleation in the binder phase forming the abovementioned core-rim structure.
During recent years many attempts have been made to control the main properties of cermets in cutting tool applications, namely toughness, wear resistance and plastic deformation resistance. Much work has been done especially regarding the chemistry of the binder phase and/or the hard phase and the formation of the core-rim structures in the hard phase. Most often only one, or at the most two, of the three properties are able to be optimized at the same time, at the expense of the third property.
U.S. Pat. No. 5,308,376 discloses a cermet in which at least 80 vol % of the hard phase constituents comprises core-rim structured particles having several, preferably at least two, different hard constituent types with respect to the composition of core and/or rim(s). These individual hard constituent types each consist of 10–80%, preferably 20–70% by volume of the total content of hard constituents.
JP-A-6-248385 discloses a Ti—Nb—W—C—N-cermet in which more than 1 vol % of the hard phase comprises coreless particles, regardless of the composition of those particles.
EP-A-872 566 discloses a cermet in which particles of different core-rim ratios coexist. When the structure of the titanium-based alloy is observed with a scanning electron microscope, particles forming the hard phase in the alloy have black core parts and peripheral parts which are located around the black core parts and appear grey. Some particles have black core parts occupying areas of at least 30% of the overall particles referred to as big cores and some have the black core parts occupying areas of less than 30% of the overall particle area are referred to as small cores. The amount of particles having big cores is 30–80% of total number of particles with cores.
U.S. Pat. No. 6,004,371 discloses a cermet comprising different microstructural components, namely cores which are remnants of and have a metal composition determined by the raw material powder, tungsten-rich cores formed during the sintering, outer rims with intermediate tungsten content formed during the sintering and a binder phase of a solid solution of at least titanium and tungsten in cobalt. Toughness and wear resistance are varied by adding WC, (Ti,W)C, and/or (Ti,W)(C,N) in varying amounts as raw materials.
U.S. Pat. No. 3,994,692 discloses cermet compositions with hard constituents consisting of Ti, W and Nb in a Co binder phase. The technological properties of these alloys as disclosed in the patent are not impressive.
A significant improvement compared to the above disclosures was presented in U.S. Pat. No. 6,344,170. By optimizing composition and sintering process using the Ti—Ta—W—C—N—Co system improved toughness and resistance to plastic deformation was accomplished. The two parameters that were used to optimize toughness and resistance to plastic deformation were Ta and Co content. The use of pure Co-based binder implied a major advantage over mixed Co—Ni-based binders with respect to the toughness behavior due to the differences in solution hardening behavior between Co and Ni. There is, however, no teaching how to optimize abrasive wear resistance simultaneously with the other two performance parameters. Hence, the abrasive wear resistance is still not optimal, which is crucial for most finishing operations.